trouble may be avoided by coating the surfaces
that it is necessary to preserve with clear liquid paste; but Mr. Renard
advises that the mould be closed hermetically, so that the interior shall
be kept from contact with warm air. In this way it is possible to prevent
the plaster from hardening, as a result of too quick a desiccation. I now
come to the operation of moulding. In the very first place, it is
necessary to examine whether it is well to adopt the arrangement by
pressure of air or by vacuum. The form of the objects will determine the
choice. A very open piece, like a bowl, must be moulded by vacuum, on
account of the difficulty of holding the closing disk in place if it be of
very large dimensions. The same is the case with large vases of wood form.
On the contrary, an elongated piece tapering from above is more easily
moulded by pressure of the air, as are also ovoid vessels 16 to 20 inches
in height. In any case it must not be forgotten that the operation by
vacuum should be preferred every time the form of the objects is adapted
to it, because this process permits of following and directing the drying,
while with pressure it is impossible to see anything when once the
apparatus is closed.

[Illustration: FIG. 2.]

_Moulding by Pressure of the Air._--The plaster mould having been put in
place upon the mould board, and the liquid paste having been long and
thoroughly stirred in order to make it homogeneous, and get rid of the air
bubbles, we open the cock that puts the paste reservoir in communication
with the lower part of the mould, care having been taken beforehand to
pour a few pints of water into the bottom of the mould. The paste in
ascending pushes this water ahead of it, and this slightly wets the
plaster and makes the paste rise regularly. When the mould is entirely
filled, the paste is still allowed to flow until it slightly exceeds the
upper level, and, spreading out over the entire thickness of the plaster,
forms a sort of thick flange. The absorption of the liquid begins almost
immediately, and, consequently, the level lowers. A new quantity of paste
is introduced, and we continue thus, in regulating its flow so as to keep
the mould always full. This operation is prolonged until the layer is
judged to be sufficiently thick, this depending upon the dimensions, form,
or construction of the vessel. The operation may take from one to five
hours.

The desired thickness having been obtained, it become